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This error does not necessarily mean that rqt failed to find and load your library file. If you are saying

Everything was working fine until I decided to add another C++ file to my project.

the lib itself is most likely found but there are some issues when loading the classes/functions in it. A very common mistake that causes this behaviour is declaring (and using) a function that is not defined (implemented); or function declaration and definition do not match. When creating an exec this would cause a linking error but as you are just creating a lib in your project (that is loaded outside your project) the linker will assume that undefined references will be resolve somewhere else when creating the lib. So always double-check all function definitions are declared and definitions and declaration match...